There are several distinct capacitor technologies, such as super capacitors and traditional electrostatic, also known as ‘dielectric’, capacitors. Super capacitors increase capacitance by increasing the surface area of the electrically conductive electrode. Most of the volume of a super capacitor is the high surface area electrode, which is basically in powder form. Increasing the super capacitor ‘depth’ while keeping constant the shape/surface area of the attaching ends, increases the amount of electrode material area and increases the capacitance. Consistent with this paradigm is the recent interest in employing graphene in super capacitors, as graphene is very good for that purpose. Indeed, graphene has very high electrical conductivity and the measured surface of some graphene forms are near the theoretical limit (2700 m2/gm). Given that the electrode surface area of graphene in super capacitors is near its theoretical limit, further dramatic energy density increases in these devices is unlikely.
In contrast to the improvements made with super capacitors, there has only been a marginal advance in the last few decades in finding materials with superior dielectric constants for the traditional electrostatic capacitor that is a capacitor built of a single dielectric material with a high dielectric constant sandwiched between two flat conductive electrodes. One clear functional contrast between super capacitors and electrostatic capacitors is the impact of ‘depth’ or ‘thickness’. In an electrostatic capacitor, capacitance increases inversely to the distance between plates. Thus, given plates of a constant size, the thinner an electrostatic capacitor, the greater the capacitance. Therefore in order to improve the performance of this style of capacitor they either have to be made thinner and/or use materials with higher and higher dielectric constants.